Passenger vehicles may include fuel cell (“FC”) systems to power certain features of a vehicle's electrical and drivetrain systems. For example, an FC system may be utilized in a vehicle to power electric drivetrain components of the vehicle directly (e.g., electric drive motors and the like) and/or via an intermediate battery system. An FC system may include a single cell or, alternatively, may include multiple cells arranged in a stack configuration.
Hydrogen is one possible fuel that may be used in a FC system. Hydrogen is a clean fuel and may be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device that includes an electrolyte disposed between an anode and a cathode. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons may be selectively conducted across the electrolyte. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water.
Proton exchange membrane fuel cells (“PEMFC”) may be used in FC vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. An anode and cathode included in a PEMFC may include finely divided catalytic particles, such as platinum (Pt), supported on carbon particles and mixed with an ionomer. A catalytic mixture may be deposited on opposing sides of the membrane.
The open circuit voltage of a typical FC stack may decrease over the lifetime of the FC stack. Voltage degradation may be attributable, among other things, to voltage cycling of the FCs in the stack. Voltage cycling occurs when the platinum catalyst particles used to enhance the electro-chemical reaction transition between a low and high potential state. The repeated transition of the catalyst particles promotes dissolution of the particles. Dissolution of the particles results in loss of active surface area of the particles and performance degradation.
Many factors may influence the relative loss in surface area of the platinum particles relating to voltage cycling, including peak stack voltage, temperature, stack humidification, voltage cycling dynamics, etc. Lower stack voltage set-points may offer greater protection against degradation, but higher stack voltage set-points may provide increased system efficiency.
A fixed voltage may be used to set a stack minimum power level to prevent unwanted voltage cycling. For example, a typical voltage suppression strategy may use a fixed voltage set-point. If a fuel cell power controller is not requesting power, or is requesting minimal power, the power generated by the stack necessary to maintain cell voltage levels at or below the fixed voltage set-point may be provided to certain systems or components where the power is used or dissipated. For example, excess power may be used to charge a high-voltage battery in a FC system vehicle. If the voltage set-point is at a relatively low voltage, then the system may charge the battery frequently, which may result in the battery charge to be at its maximum charge more frequently than it would be if the voltage set-point is set at a lower level. If the battery is at its maximum charge and cannot accept more charging power from the FC system, then the controller may cause the excess power to be dissipated in other components, such as resistors. Where excess power is dissipated using resistors, system efficiency may suffer. Accordingly, establishing an optimal set-point may improve the efficiency of an FC stack.
Several FCs may be combined in an FC stack to produce a desired power out. The FC stack may receive a cathode input gas that may comprise a flow of air forced through the stack by a compressor. Cathode exhaust gas may include water as a stack by-product, together with unconsumed oxygen and other gases.
The current/voltage relationship of the stack may be referred to as a polarization curve. A stack controller may utilize information about the polarization curve to schedule delivery of reactants to the FC system in accordance with power demands. The relationship between the voltage and the current of the stack may be non-linear, and may be influenced by many variables, including stack temperature, stack partial pressures, and cathode and anode stoichiometries. Additionally, the relationship between the FC stack current and voltage may change over time as the stack performance degrades. In other words, an older FC stack may have lower cell voltages, and accordingly, an older FC stack may need to provide more current than a new, non-degraded stack to produce an equivalent power output.